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1. Field of the Invention
The invention generally relates organic liquid and polymer electrolytes for rechargeable batteries and electrochemical capacitors. More particularly, the invention relates to organosilicon ammonium derivatives for use as electrolyte additives.
2. Background
The demand for new and improved electronic devices such as cellular phones, notebook computers and compact camcorders has resulted in demand for energy storage devices having increasingly higher specific energy densities. A number of advanced battery technologies have recently been developed to service these devices, such as metal hydride (e.g., Nixe2x80x94MH), lithium batteries with liquid electrolytes and recently, lithium batteries with polymer electrolytes.
Lithium batteries have been introduced into the market because of their high energy densities. Lithium is atomic number three on the periodic table of elements, having the lightest atomic weight and highest energy density of any solid material. As a result, lithium is a preferred material for batteries, having very high energy density. Lithium batteries are also desirable because they have a high unit cell voltage of up to approximately 4.2 V, as compared to approximately 1.5 V for both Nixe2x80x94Cd and Nixe2x80x94MH cells.
Lithium batteries can be either lithium ion batteries or lithium metal batteries. Lithium ion batteries intercalate lithium ions in a host material, such as graphite, to form the anode. On the other hand, lithium metal batteries use metallic lithium for the anode.
The electrolyte used in lithium batteries can be a liquid or a polymer electrolyte. Lithium batteries having liquid electrolytes have been on the market for several years. Lithium batteries having solid polymer electrolytes are comparatively new entries into the marketplace.
The electrochemical operation of a lithium battery is essentially the same whether a liquid electrolyte or polymer electrolyte is used, and is based on the anode and cathode materials used. In the case of a lithium ion battery, the battery works by the rocking chair principle, that is, charging and discharging, allowing lithium ions to xe2x80x9crockxe2x80x9d back and forth between cathode and anode and for lithium ions to be involved with the intercalation-deintercalation process on the active electrode material surfaces.
During the cycling of a lithium-metal battery the following processes occur. While discharging, lithium dissolution takes place at the metal lithium anode, and results in passing lithium ions into the electrolyte. On the cathode during discharging, intercalation of lithium ions into the solid phase occurs. During the charging of a lithium-metal battery, lithium cations deintercalate from the solid phase cathode, and the deposition of metal lithium takes place on the metal lithium anode from lithium ions in the nonaqueous liquid electrolyte.
Many performance parameters of lithium batteries are associated with the electrolyte choice, and the interaction of the selected electrolyte with the cathode and anode materials used. Most devices require electrolytes to provide high conductivity and electrochemical stability over a broad range of temperatures and potentials. The electric conductivity (specific and molecular or equivalent) is one of the most important properties of electrolytes. High electrolyte ionic conductivity leads to improved battery performance. Thus, significant research has focused on developing methods for increasing electrolyte conductivity and also its chemical and electrochemical stability in electrochemical cells.
Electrolytes generally include one or more aprotic solvents, at least one salt and may also contain optional electrolyte additives. The ionic conductivity of such systems is substantially determined by interaction between the salt and the solvent and by the resulting values of the ion mobilities in liquid electrolyte systems and ion mobilities in the intermolecular spaces for plasticized polymer electrolyte systems. When selecting the composition of the electrolyte system it is generally necessary to choose solvents with a low viscosity.
One known way to increase the ionic conductivity of electrolyte solutions is through the use of mixed solvents. Using laws of ion dissociation, dielectric permeability and viscosity applied to electrolyte conductivity, it is possible to select improved compositions of solvent and solution. Thus, theoretical considerations make it possible to select solvents which provide a high constant for electrolytic dissociation in nonaqueous media.
One of the components of the mixed solvent can be regarded as a solvating agent, which supplies the system with solvation energy necessary for the appropriate ion pair formation. The second component determines a sufficiently high dielectric permeability, which is essential for the disintegration of ion pairs with the formation of free solvated ions. It is well known that in mixed solvents, alkali metal halogenides dissociate much better than in each solvent component separately.
Modifying additives can be effective in increasing the ionic conductivity and stability of nonaqueous electrolytes. Additives vary in both their chemical nature and the mechanisms of their influence. Among these additives, nitrogen containing ones, such as ternary amines and certain ammonium compounds, have been reported to produce improved electrolytes. Ternary amines have been shown to be capable of considerably increasing the solvating activity of aprotic solvents. Ternary amines in an electrolyte PEO4 (polyethylene oxide) with acrylonitrile and LiCl (or LiBr or LiI) have been shown to increase the Li ion conductivity by up to two orders of magnitude [X. Q. Yang, H. S. Lee, J. McBreen, L. S. Choi, Y. Okamoto. The Ion Pair Effect of Aza-based Anion Receptors on Lithium Salts in Polymer Electrolytes, In Proceeding Fall Meeting, San-Antonio, Tex., Oct. 6-11, 1996, Meeting abstract, Abstr. N76]. The mentioned ternary amines were introduced into the solution in equimolar amounts in relation to the lithium salts. This increase in conductivity can be partly attributed to the formation of anion complexes (Clxe2x88x92, Brxe2x88x92, I) with the nitrogen containing additives and the formation of complexes of Li+ ions with ether oxygen in PEO4. These processes make the dissociation of lithium salts into ions more effective.
The use of ternary amines, in particular tributylamine as additives into 1,3-dioxolane and LiAsF6 based nonaqueous electrolytes have been reported (D. Aurbach, E. Zinigrad, H. Teller, P. Dan, J. Electrochem. Soc., 147 (4) 1274-1279 (2000)). As far as the modifying activity is concerned, the authors related the influence of tributylamine to its antipolymerization activity preventing the polymerization of 1,3-dioxolane. Such an approach is effective when fluorine containing lithium salts like LiBF4, LiAsF6 and LiPF6 are used in nonaqueous electrolytes as ion-conducting additives. During operation, the lithium salts breakdown forming the inorganic acid HF. This acid in turn initiates the polymerization of the organic solvent. Thus, stabilization of the electrolyte""s properties is obtained at the expense of HF acceptance by ternary amines.
Even with available electrolyte additives, conventional electrolytes for lithium secondary batteries do not provide sufficient ionic conductivity for many applications and/or are not stable enough for most applications with lithium metal secondary batteries or lithium ion secondary batteries. This is principally because previous additives have been directed at improving only one aspect of electrolyte performance, such as ionic conductivity or cycling efficiency. Moreover, conventional electrolytes cannot generally provide a lithium secondary battery having satisfactory cycling characteristics, such as charge-discharge efficiency, cycle lifetime and the like.
A group of pyridinium based compounds includes an organosilicon backbone containing at least one ethylene oxide (CH2CH2O) unit, and at least two pyridinium groups bound to the backbone, the pyridinium groups each bound at least one halogen ion or halogen-containing anion. As used herein, a pyridinium group is defined as by the following general structure: 
where Xxe2x88x92 is a halogen ion or a halogen containing anion, R2 and R3 are selected from hydrocarbons, hydrogen or nitrogen containing heterocyclic substituents.
The compounds can be use as an additive for forming improved liquid and polymer electrolytes and for the formation of lithium ion and lithium metal batteries having enhanced properties. When used to form a nonaqueous liquid electrolyte, the electrolyte is formed by combining the additive with an aprotic solvent. When used to form a nonaqueous polymer electrolyte, the electrolyte is formed by combining the additive with a polymer matrix and an aprotic solvent.
The electrolytes may include other optional components. The additive can have the following general structure: 
where n is an integer from 1 to 9, Xxe2x88x92 is preferably selected from Clxe2x88x92, Brxe2x88x92, lxe2x88x92, ClO4xe2x88x92, BF4xe2x88x92, AsF6xe2x88x92, PF6xe2x88x92, CF3SO3xe2x88x92, CF3CF2CF2SO3xe2x88x92 and N(CF3SO2)2, R1 are aliphatic or aromatic hydrocarbons, R2 and R3 are hydrocarbons, hydrogen or nitrogen containing heterocyclic substituents (e.g., C5H4N (pyridine)). Preferably, R1, R2 and R3 are selected in the following combinations:
a) R1=CH3, R2=R3=H;
b) R1=CH3, R2=2-CH3, R3=H;
c) R1=CH3, R2=4-CH3, R3=H;
d) R1=CH3, R2=5-(CH2xe2x95x90CHxe2x80x94), R3=2-CH3;
e) R1=CH3, R2=4-(CH2xe2x95x90CHxe2x80x94), R3=H;
f) R1=CH3, R2=4-C5H4N, R3=H;
g) R1=C6H5, R2=R3=H;
h) R1=C6H5, R2=2-CH3, R3=H;
i) R1=C6H5, R2=4-CH3, R3=H;
j) R1=C6H5, R2=5-(CH2xe2x95x90CHxe2x80x94), R3=2-CH3;
k) R1=C6H5, R2=4-(CH2xe2x95x90CHxe2x80x94), R3=H; and
l) R1=C6H5, R2=4-C5H4N, R3=H.
In a more preferred embodiment, the additive has the following general structure: 
Liquid and polymer electrolytes can include the salt of an alkali metal. The salt can be selected from the group consisting of LiClO4, LiBF4, LiAsF6, LiCF3SO3 and LiN(CF3SO2)2.
Aprotic solvents for liquid and polymer electrolytes can include acetonitrile, dimethylformamide, dimethylsulfoxide, propylene carbonate, ethylene carbonate, dimethyl carbonate, bis(2-methoxyethyl) ether, gamma butyrolactone, 1,3-dioxolane, dimethoxyethane or sulfolane. In this embodiment, 0.05 to 5 weight percent of the electrolyte is preferably provided by the additive.
In the polymer electrolyte embodiment, the aprotic solvent can comprise 40 to 82 weight percent of the electrolyte, while polymeric matrix can comprise 10 to 40 weight percent of the electrolyte. The polymer matrix can be a halogen-containing polymer. For example, the halogen-containing polymer can be polyvinyl chloride, chlorinated polyvinyl chloride or polyvinylidene fluoride.
An electrochemical cell includes an alkali metal or alkali ion containing anode, a cathode, and the additive containing the electrolyte of claim 1 or the additive containing electrolyte including the polymer matrix of claim 8. The electrochemical cell can be a rechargeable cell. If the anode is a lithium metal anode, the lithium metal anode can be lithium alloy. In this embodiment, the lithium alloy can be lithium-aluminum, lithium-aluminum-silicon, lithium-aluminum-cadmium, lithium-aluminum -bismuth or lithium-aluminum-tin.
In the lithium ion embodiment, the anode can include carbon. The carbon is preferably graphite.
The cathode for the lithium metal cell can be a metal oxide, such as MnO2, CuO, V2O5, V6O13 or TiS2. The cathode for the lithium ion cell can be a metal oxide, such as LiMn2O4, LiCoO2, LiNiO2 or LiVxOy.
A method of forming a liquid electrolyte includes the steps of providing an aprotic solvent, the salt of an alkali metal and an organosilicon additive. A method of forming a polymer electrolyte includes the steps of providing composition a polymeric matrix, an aprotic solvent and the salt of an alkali metal and an organosilicon additive. The additive includes an organosilicon backbone including at least one ethylene oxide (CH2CH2O) unit and at least two pyridinium groups bound to the backbone, the pyridinium groups each bound to at least one halogen ion or halogen-containing anion. The above components are then mixed together.
A method of making an electrochemical cell includes the steps of providing an anode including an alkali metal or alkali ion and providing a cathode of an electrochemically active material. A nonaqueous liquid electrolyte is placed between the anode and the cathode with using a porous separator, such as a polypropylene separator. A nonaqueous polymer electrolyte is operatively associated with the anode and cathode. The nonaqueous liquid and polymer electrolyte includes at least one ion-forming alkali salt. An organosilicon additive is combined into the electrolyte. The additive includes an organosilicon backbone including at least one ethylene oxide (CH2CH2O) unit and at least two pyridinium groups bound to the backbone, the pyridinium groups each bound to at least one halogen ion or halogen-containing anion.